1. Field of the Invention
The present invention relates to an improved method of charging molten metal into the mold cavity of a vertical die casting machine.
2. Description of Relevant Art
A widely known method of charging molten metal into a vertical die casting machine comprises the steps of: feeding molten metal onto a lower plunger in the raised position thereof; lowering the lower closure plunger synchronously with an upper plunger, in accordance with a direction for lowering the lower plunger, to thereby open a runner opening; and injecting the molten metal into a mold cavity. It is important in connection with such injection to lower the lower plunger synchronously at a predetermined timing and speed in accordance with the operation of the upper plunger. However, it is very difficult to synchronize the lower plunger with the upper plunger precisely for each injection, because variations arise in the amount of metal poured, the pre-set level of timing, etc. Such a known method is disclosed, for example, in British Patent Specification No. 843,959 published on Aug. 10, 1960.
If a large amount of molten metal is poured, for example, the upper plunger is brought into contact with the molten metal before the direction to lower the lower plunger is given. Pressure develops in the molten metal sleeve, and causes a part of the metal to enter the clearance between the sleeve and the plunger where they are in sliding contact with each other. This results in undesirable spattering of a part of the molten metal out of the machine, or solidification of the metal in the area between the sleeve and the plunger, adversely affecting the operation of the plunger. Because the molten metal comes to be under a high pressure, it spurts out from the runner opening when the opening is opened, and is extremely difficult to charge into the mold cavity smoothly.
If the amount of molten metal is too small, the upper plunger does not exert pressure on the molten metal until after the lower plunger opens the runner opening to permit the molten metal to flow into the mold cavity. Thus, smooth charging cannot be expected in such case. Also, because the upper plunger is brought into contact with the molten metal with some delay after the metal has flowed into the mold cavity, and causes the metal to flow into the mold cavity again, there arises a number of defects having an adverse affect on the quality of the casting produced. Such defects include, for example, cold shut, inclusion of air forming cavities, and shrinkage cavities formed upon solidification. According to the commonly-known die casting operation, it is difficult to pour additional molten metal to compensate for solidification shrinkage and to provide solidification under pressure because the operation normally involves a high rate of solidification and a narrow sprue.
A further disadvantage attendant the conventional method described hereinabove resides in the complicated construction of the synchronous driving mechanisms for the upper and lower plungers. The mechanisms are expensive, with a resultant increase in the overall cost of production, which cost is further increased due to the reduction in the yield of production caused by the above set forth disadvantages.
The present invention provides an effective solution to the foregoing various problems attendant the conventional method of charging molten metal under pressure into a vertical die casting machine.